Nested interbody spinal fusion implants

ABSTRACT

Tapered root threaded hollow perforated interbody spinal fusion implants are disclosed for placement into a disc space in a human spine between adjacent vertebral bodies. The implants have opposite arcuate portions with lockable screws passing therethrough for engaging each of the adjacent vertebral bodies. The implants are adapted for use in side-by-side pairs such that a portion of the circumference of a first implant nests within the circumference of a second implant, so as to have a reduced combined width.

RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/825,522, filed Apr. 15, 2004; which is a divisional of U.S.application Ser. No. 10/246,931, filed Sep. 19, 2002 (now U.S. Pat. No.7,637,951); which is a divisional of U.S. application Ser. No.09/566,272, filed May 5, 2000 (now U.S. Pat. No. 6,485,517); whichclaims the benefit of U.S. Provisional Application No. 60/132,665, filedMay 5, 1999; all of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION Description of the Related Art

The use of hollow threaded perforated interbody spinal fusion implantssuch as taught by U.S. Pat. No. 5,015,247 to Michelson ('247),incorporated by reference herein, is now commonplace. Nevertheless,because of the structure and biomechanical properties of these implants,the use of such implants has not been available for all patientsrequiring spinal fusion, but rather has been limited to a subset of thatpopulation. While such implants have proven to be very successful whenused correctly, such success has not been universal.

A previously identified problem as discussed in U.S. Pat. No. 4,593,409to Michelson ('409), incorporated by reference herein, is the frequentneed for such implants to have a reduced combined width relative totheir combined height. This permits the height, which is usually theimplant diameter, to be sufficiently great so as to span the height ofthe distracted disc space and adequately penetrate and engage each ofthe vertebral bodies adjacent that disc space, and yet have asignificantly lesser width so that when such implants are utilized inside-by-side pairs, the combined width is such that the paired implantsdo not protrude beyond the width of the spine. Historically, this notinfrequent situation has deprived many patients needing spinal fusionfrom use of the prior art technology as implants of the desired heightcould not safely be placed within that patient's disc space because ofthe width problem. Alternatively, downsized versions of these implantswere implanted with poor results as the implants were of insufficientsize to adequately function for their intended purpose.

As discussed in Michelson '409, implants having various vertebral boneengaging surface projections have the advantage of enhanced stabilitywithin the spine as compared to an implant having a smooth surface. Theuse of a thread or thread portions has proven particularly beneficialand have been described in Michelson '247. As described in Michelson'sco-pending application Ser. Nos. 08/484,928; 08/480,904; and 08/480,908incorporated by reference herein, similar devices in which oppositevertebrae engaging arcuate surfaces are in angular relationship to eachother may be useful to be fuse the vertebrae in a more lordotic angularrelationship relative to each other. Implants of the related art aretaller near the end adapted to be placed proximate the anterior aspectof the vertebral bodies than at the opposite end adapted to be placedproximate the posterior aspect of vertebral bodies. The related artimplants are generally wedge-shaped when viewed from the side. Thewedged configuration causes the implant to be less stable within thespine than if it were non-wedged. Further limiting the stability ofthese implants, compromising the surface area available for contact andfusion, and limiting the volume of osteogenic material containablewithin the implants is a result of the fact that these implants havegenerally been relatively flat across their trailing ends so as to berotationally symmetrical about their mid-longitudinal axes. The anterioraspects of the vertebral bodies are generally curved from side-to-side.As a result, related art implants needed to be rather deeply inset intothe disc space and away from the anterior aspects of the vertebralbodies so as to prevent the implants from protruding from the disc spaceat their lateral wall and trailing end junctions, wheresuch a protrusionof the implant could place vital structures adjacent the spine at risk.

There is, therefore, a need for further improvement in the design ofsuch interbody spinal fusion implants so as to firstly extend theirrange of usefulness, and secondly to further increase the rate ofsuccess when such implants are used.

SUMMARY OF THE INVENTION

In accordance with the present invention, as embodied and broadlydescribed herein, there are provided interbody spinal fusion implantsthat are threaded at least in part and require an element of rotationfor insertion across a disc space between two adjacent vertebral bodiesof a spine. The implants of the present invention are configured to bepositioned in close proximity to each other such that the combined widthof the implants is less than the combined height of the implants. Theimplants preferably have a leading end, a trailing end opposite theleading end, and a mid-longitudinal axis and length therebetween. Theimplants preferably have opposite arcuate portions adapted for placementtoward and at least in part within the adjacent vertebral bodies andhave a distance therebetween defining an implant height greater than thenormal height of the disc space to be fused. Each of the oppositearcuate portions preferably has at least one opening in communicationwith each other for permitting for the growth of bone from vertebralbody to adjacent vertebral body through the implant. Preferably, atleast a portion of a thread is formed on the exterior of each of theopposite arcuate portions for penetrably engaging the adjacent vertebralbodies and to facilitate securing the implant into the spine by at leastin part rotating the implant about its mid-longitudinal axis. At least afirst one of the implants preferably has a lateral side wall and amedial side wall with a distance therebetween defining an implant widthtransverse to the implant height. The width of the first implant is lessthan its height along at least a portion of its length. The medial sidewall of the first implant is preferably configured to be positioned inclose proximity to at least a second spinal implant such that thecombined width of the first and second implants is less than thecombined height of those implants.

The present invention provides for improved interbody spinal fusionimplants for placement within the spine in longitudinal side-by-sidenested pairs. As used herein, the terms “nesting or nested” refer to theplacement of at least two implants in side-by-side relationship andclose proximity to each other. In a preferred embodiment, the presentinvention teaches the nesting together of a pair of tapered rootthreaded spinal fusion implants, such that the nested implant pair has acombined reduced width relative to the combined height of the individualimplants. As used herein, the terms “tapered root” refers to an implanthaving an outer diameter as measured at the peaks of the bonepenetrating protrusions, such as threads, and a root diameter, whereinthe root diameter tapers from one end to the other end of the implant.

An embodiment of the present invention includes an interbody spinalfusion implant adapted to receive along its length a secondcircumferentially threaded complimentary interbody spinal fusionimplant, such that the second implant nests within the circumference ofthe first implant. The nested longitudinal side-by-side pair has acombined width less than the implants' combined maximum diameters, whichmaximum diameters generally define the over-all implant heights. In afurther embodiment, the implants of the present invention are angledtoward each other such that the combined width at the leading ends isfurther lessened.

The present invention implants have opposite arcuate portions andpreferably are rotated into place. In an embodiment of the presentinvention, the implants may be generally cylindrical and have a threador thread portions. In a preferred embodiment, the root diameter of theimplant is generally conical or a portion of cone in that the oppositearcuate surfaces for contacting the vertebrae adjacent the disc spaceare in angular relationship to each other generally over the length ofthe implants. In a preferred embodiment of a tapered root implant, theouter diameter of the implant as measured at the thread peaks remainsrelatively constant over the length of most of the implant. As the rootdiameter of the implant tapers down, the thread height increases suchthat the outer diameter of the implant as measured at the thread peaksremains relatively constant.

The present invention has at least one of a pair of implants having atleast one side adapted to receive within the over-all circumference ofthe outer diameter of the implant the side of a second implant. In apreferred embodiment, the receiving implant has both leading andtrailing support walls, and while preferable, but not requisite, thesewalls may provide structural support and nevertheless be highlyperforated to allow for vascular access and the growth of bone throughthe implant. In a preferred embodiment of a second implant to bereceived within the first implant, the leading end support structurefurther comprises a cap, which cap need not, but may be threaded, andwhich cap need not be, but preferably is perforated.

In a preferred embodiment of the present invention, the trailing ends ofthe implants are rotationally asymmetrical about the mid-longitudinalaxis such that they may be inserted in nested fashion and in properrotational alignment relative to each other and to the vertebral bodies,with the result that the implants will have a length along the lateralaspect from leading to trailing end less than the length along themedial wall from leading end to trailing end. Preferably, the implantsof the present invention are structurally adapted, such that whenproperly inserted, the length of the lateral side wall as measured fromthe leading end to the trailing end is of a lesser length than thelength of the implant along its mid-longitudinal axis so as to preventthe protrusion of the lateral side wall and trailing end junction beyondthe circumferentially curved profile of the vertebral bodies.

While the present invention does not require it, in a preferredembodiment the implants are adapted to receive through their trailingends opposed bone screws and to transmit at least threaded portions ofthose screws through the opposite vertebrae engaging arcuate portions soas to allow those bone screws to engage at least one each into each ofthe vertebral bodies adjacent a disc space into which the implant areimplanted.

Each of the embodiments of the implants of the present invention mayalso include one or more of anatomically contoured trailing ends,tapered minor diameters, opposed bone engaging screws, and locks forlocking the opposed bone engaging screws into place. In a preferredembodiment, the implants of the present invention are configured toreceive bone screw locks to lock the opposed bone screws to theimplants. The bone screws are preferably lag screws and the locks, whilepreventing the backing out of the lag screws from the implant, mayeither be rigidly fixed or allow for continuing angular motion of thelag screws relative to the implants.

The present invention also is directed to an improved method forinserting such implants.

The accompanying drawings, which are incorporated in and constitute apart of this specification, are by way of example only and notlimitation, and illustrate several embodiments of the invention, whichtogether with the description, serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of the lateral aspect of the humanspine with implants of an embodiment of the present invention implantedtherein.

FIG. 2 is a front perspective view of two adjacent lumbar vertebrae andtwo implants of an embodiment of the present invention implanted acrossthe disc space.

FIG. 3A is a top plan view of the implants of FIG. 2 and a cross sectionthrough a vertebra along line 3A-3A of FIG. 2.

FIG. 3B is a top plan view of another embodiment of the implants of FIG.3A and a cross section through a vertebra along line 3A-3A of FIG. 2.

FIG. 3C is a top plan view of another embodiment of the implants of FIG.3A and a cross section through a vertebra along line 3A-3A of FIG. 2.

FIG. 4A is a top plan view of the implants of FIG. 3A.

FIG. 4B is an end view of another embodiment of an implant of thepresent invention with two implants shown in hidden line.

FIG. 5 is a side perspective view of an embodiment of an implant of thepresent invention.

FIG. 6 is a side perspective view of another embodiment of an implant ofthe present invention.

FIG. 7 is a side elevational view of the implant of FIG. 5.

FIG. 8 is a leading end view of the implant of FIG. 6.

FIG. 9 is a front elevational view of two adjacent vertebrae in a lumbarspine with the implants of one embodiment of the present inventionimplanted across the disc space therebetween.

FIG. 10 is a cross sectional side elevational view of the lateral aspectof two adjacent lumbar vertebrae illustrating the bore created by adrill.

FIG. 11 is a cross sectional side elevational view of the lateral aspectof two adjacent vertebrae and an embodiment of an implant of the presentinvention inserted therebetween.

FIG. 12 is a cross sectional side elevational view of the adjacentvertebrae and implant of FIG. 11 with a screw driver and a bone engagingscrew being installed.

FIG. 13 is a side elevational cross sectional view of the adjacentvertebrae and the implant of FIG. 11 with a bone engaging screw in theinstalled position.

FIG. 14 is a side elevational cross sectional view of the adjacentvertebrae and implant of FIG. 11 with two bone engaging screws in theinstalled positioned.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is intended to be representative only and notlimiting and many variations can be anticipated according to theseteachings, which are included within the scope of the present invention.Reference will now be made in detail to the preferred embodiments ofthis invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 1 is a side elevational view of the lateral aspect of a humanlumbar spine S having vertebral bodies V and discs D interposedtherebetween. The trailing end of an implant 100 can be seen located inthe disc space between the fourth and fifth lumbar vertebrae L₄ and L₅wherein disc space D has been distracted more anteriorly thanposteriorly.

FIG. 2 is a front perspective view of adjacent vertebrae within a lumbarspine designated as V₁ and V₂. Interposed therebetween is a disc D, withpaired implants 100 and 200 in nested side-by-side opposition insertedin the disc space. Implants 100 and 200 have heights greater than theheight of the restored disc space into which they are implanted so as toexpand the height of that disc space and to further penetrably engageinto the bone of each of vertebral bodies V₁, V₂. In a preferredembodiment, the circumference of the trailing end of implant 100preferably extends within the circumference of implant 200 when implants100, 200 are in nested, longitudinal side-by-side placement.

As shown in FIGS. 3A, 4A, and 5-8, implants 100, 200 preferably eachhave a leading end 108, 208 for insertion first into the disc space; atrailing end 104, 204, opposite leading ends 108, 208, respectively; anda mid-longitudinal axis and length therebetween. Implants 100, 200preferably each have upper and lower opposite arcuate portions 134,134′, and 234, 234′ adapted for placement toward and at least in partwithin adjacent vertebral bodies V₁, V₂ and have a distance therebetweendefining an implant height greater than the normal height of the discspace to be fused.

Preferably, opposite arcuate portions 134, 134′ and 234, 234′ each haveat least one opening 120, 220 communicating with one another forpermitting the growth of bone from vertebral body to adjacent vertebralbody through the implant. Preferably, at least a portion of a bonepenetrating protrusion, such as thread 116, 216 is formed on theexterior of each of the opposite arcuate portions 134, 234,respectively, for penetrably engaging the adjacent vertebral bodies andto facilitate securing the implant into the spine by at least in partrotating the implant about its mid-longitudinal axis.

FIGS. 5-8 show that in a preferred embodiment, implants 100, 200 have abody which is generally frusto-conical, increasing in diameter fromleading end 108, 208 to trailing end 104, 204 and preferably has ahelical thread 116, 216 about its circumference. Thread 116, 216 canhave a generally constant outside diameter and can progress from a sharppointed profile at the thread portion proximate leading end 108, 208 toa thicker and more squared profile toward trailing end 104, 204. Thread116, 216 may be interrupted as shown in FIGS. 5 and 6. These specificsare shown by way of example only and not limitation. It is appreciatedthat implants 100, 200 may have any type of thread or threads useful forthe intended purpose without departing from the present invention.Further, the implants of the present invention need not befrusto-conical as shown, and could be shaped much like a cylinder cut inhalf transversely through its mid-longitudinal axis with the upper andlower halves of the implant in angular relationship to each other.

In a preferred embodiment, the root diameter of the implants of thepresent invention tapers from its trailing end for placement anteriorlyto its leading end for placement posterior within a disc space. Thisconfiguration is particularly desirable for providing for the properlordotic relationship between the adjacent vertebral bodies, such thatthose vertebral bodies are distanced apart greater anteriorly thanposteriorly. In a further preferred embodiment of these lordoticimplants, the thread has a generally constant outside diameter such thatin combination with the tapered root the actual thread height increasesfrom the trailing end to the leading end progressively. At the leadingend the thread may have a lesser height for facilitating the implantinsertion without departing from this teaching.

FIGS. 5 and 7 show a preferred embodiment of implant 200. Lateral sidewall 228 and medial side wall 232 have a distance therebetween definingan implant width transverse to the implant height. The width of implant200 is less than its height along at least a portion of its length.Medial side wall 232 is preferably configured to be positioned in closeproximity to at least implant 100 such that the combined width ofimplants 100, 200 is less than the combined height of those implants.

Implant 200 is similar to implant 100, but differs from implant 100 inthat while the lateral sides 128 and 228 of implants 100 and 200,respectively, are the same and in this example convex, the medial side232 of implant 200 has been relieved so as to allow for the convexmedial side 132 of implant 100 to protrude therein. Alternatively,medial side of implant 200 can be relieved, in part absent, and/orconcave.

Implant 200 also has at medial side 232 a convexity as shown by thecontour of trailing support wall 260. In a preferred embodiment, leadingsupport wall 256 may similarly be concave. And further a portion of themedial side wall 232 is absent so as to allow for the protrusion ofimplant 100 therein.

As shown in FIG. 4B, in another embodiment of the present invention, animplant 300 may have a lateral side wall 328 and an opposite medial sidewall 332. Both lateral side wall 328 and medial side wall 332 can beconfigured to be positioned in close proximity to two implants 100, oneon each side of implant 300. The combined width of implant 300 and twoimplants 100 is less than the combined height of those implants.

Without departing from the scope of the present invention, the medialside of the implant need not be present as a single large opening 252and could have a wall or support portion to it. In a preferredembodiment as shown, the large opening 252 allows for the easy packingof the implant with such osteogenic material as bone or a carriercontaining bone morphogenetic protein or genetic material coding for theproduction of bone. A further advantage of the openness of this area isthat it allows for further collateral vascularization to support bonegrowth from implant to implant. Again, while the present invention isnot limited to a medial opening as shown, implant wall edge 268 can becontinuous longitudinally to provide for strength in this critical areaand if desired can be sharpened as can interrupted thread bases 272 soas to further facilitate the self-tapping nature of these implants.

In a preferred embodiment of the present invention, leading ends 108,208 and trailing ends 104, 204 are also perforated. While leading end208 could be more or less open, in a preferred embodiment there is astructural support portion 256 perforated by openings herein shown as276. Trailing ends 104, 204 preferably have a plurality of openings 144,244 therethrough to allow for vascular access into implants 100, 200 andthe possibility of bone growth therethrough. Openings 144, 244 may alsocommunicate with further openings such as 148, 148′, 248, 248′ tofurther increase the porosity of the trailing end of the implant.Implant 200 preferably is both hollow and highly perforated.Alternatively, the implants of the present invention can comprise aporous type of material such as a cancellously structured tantalum.

As can be appreciated from FIGS. 3A-4A, implant 100 is preferablystructurally configured so as to cooperatively interdigitate into themaximum circumference of implant 200 along its length. When implant 100is inserted within the circumference of implant 200 as shown in FIGS.3A-4A, the combined width of implants 100 and 200 is substantially lessthan their combined heights, which heights in this case correspond tothe maximum circumference of each of the implants which in this exampleis the same.

While the implants of the present invention can have any of a variety ofconfigurations at their trailing ends, trailing end 104 of implant 100,and trailing end 204 of implant 200 are preferably contoured to sit onthe anterior rim AR of vertebral body V₁. As shown in FIG. 3A, trailingends 104, 204 are asymmetrical about the mid-longitudinal axis of theimplants 100, 200, respectively. In the final installed position of theimplants, trailing ends 104, 204 preferably and generally conform to atleast a portion of the anatomic curvature of the anterior rim AR ofvertebral bodies V_(1,2).

As shown in FIG. 3B, alternatively, trailing end 104′ of implant 100′may be symmetrical about the mid-longitudinal axis of implant 100′ andtrailing end 204′ of implant 200′ may be asymmetrical about themid-longitudinal axis of implant 200′. Trailing ends 104′, 204′ arepreferably contoured to sit on the anterior rim AR of vertebral body V₂.

As shown in FIG. 3C, alternatively, trailing end 104″ of implant 100″and trailing end 204″ may be symmetrical about the mid-longitudinal axesof implants 100″, 200″, respectively. Trailing ends 104″, 204″ arepreferably contoured to sit on the anterior rim AR of vertebral body V₂.

As best shown in FIG. 8, leading end 108 of implant 100 comprises anexternally threaded cap 112 for threading into the internally threadedopening of leading end 108. Cap 112 is rotatable in either direction foropening and closing of implant 100 by cap 112 with hex opening 172 whichcan be manipulated with a hex driver. The specifics of cap 112, such asthe bone holes 176, are shown by way of example only and not limitation.Cap 112 is useful for allowing for the compressive loading of theimplant with osteogenic materials such as bone, demineralized bonematrix, carriers such as collagen or any other material useful as acarrier for the delivery of bone producing materials such asmorphogenetic proteins, mineralizing proteins, or chemical compounds orgenetic material coding for the production of bone. The cap itself maybe more or less perforated and the opening themselves can be eitherlarger or smaller, less or more, as desired to provide access to theinterior of the implant, bone growth therethrough, and to the extentdesired acts so as to contain the osteogenic material generally withinthe implant and from grossly expelling out of the leading end. The capmay be used as a structural member adding strength to the implantoverall. Depending on the specific qualities desired, the cap can bemade of any material appropriate for its purpose. Such materials wouldinclude various plastics, including polyethylene, and may includebioresorbable plastics as well. Such materials may include corticalbone, ceramic, or any surgical quality metal suitable for the intendedpurpose and including by way of example only and not limitation,surgical grade titanium and its alloys.

FIG. 4A is a top view of nested implants 100, 200 of FIG. 3A. As shownin FIG. 4A, the implants of an embodiment of the present invention arethreaded on their exterior surface. While the embodiments shown havebone penetrating projections in the form of a helical thread, thepresent invention allows for a continuous thread, or an implant havingthread portions such as would be useful for an implant to be inserted bylinear advancement and then requiring an element of rotation, forexample 90 degrees, to set the thread portions into the bone of theadjacent vertebral bodies.

As an alternative, the bone penetrating projecting segments need not beportions of a thread, but rather could be concentric projectionsgenerally, but not necessarily, oriented perpendicular to the long axisof the implants. For example, the projection may be a ridge or a finadapted to penetrably engage the bone of the adjacent vertebral bodies.

As shown in FIG. 5, thread 116 of implant 100 may have a generallyconstant outer diameter. Inasmuch as the body of implant 100 isgenerally conical such that it tapers from the larger trailing end 104to the smaller leading end 108, the height of thread 116 relative tobody 102 increases from trailing end 104 to leading end 108. Thus, whilethe outer diameter of the threads remains generally constant, the heightof the thread increases from trailing end 104 to leading end 108. Thisis similarly true for implant 200.

In a preferred embodiment of implant 100, 200 the start of the externalthread about the perimeter of the implant is precisely indexed such thatif the surgeon knows the depth of the bore created, he may select animplant of the desired length being less than or equal to the depth ofthe bore created and by starting the insertion of the implant in apreferred rotational alignment such as the desired final rotationalalignment the implant when threaded in fully will come to rest such thattrailing end 104, 204 will be correctly rotationally aligned so thatscrew receiving holes 236, 236′ will be oriented correctly towards theadjacent vertebral bodies while the profile of trailing ends 104, 204will correspond to the contour of the anterior vertebral body.

By way of example, for a bore measured to receive a 30 millimetermaximum length implant having a pitch of three millimeters as anexample, the start of the thread at the implant leading end could beindexed such that the implant could be introduced rotationally orientedexactly as desired for the final positioning. Then, by making tencomplete revolutions of three millimeters each the implant wouldassuredly come to rest with trailing wall 204 appropriately oriented andeither be flush with the anterior vertebral cortices, or minimallycounter-sunk to exactly the extent to which the surgeon caused theimplant to enter the bore prior to initiating rotation. As previouslymentioned, trailing end 204 of implant 200 could be rotationallyasymmetrical, but nevertheless be symmetrical from side-to-side, suchthat each of the sides of the implant would be less protuberantposteriorly than a point along the mid-longitudinal axis such that theimplant could be correctly inserted in increments of less than or equalto 180 degrees of rotation.

While a preferred embodiment of the present invention is directed to atapered root implant having a constant outer diameter thread, thepresent invention is not so limited. The present invention also wouldinclude implants having bodies that are more or less cylindrical. Thepresent invention also would include other thread designs including athread having a constant height relative to the body and with or withouta constant outer diameter depending on the shape of the body.

As shown in FIG. 4A, extending through both upper and lower surfaces134, 234 and located approximate trailing ends 104, 204 of implants 100,200 are openings 124, 124′, and 224, 224′ respectively, for transmittingthe threaded shafts of bone screws for engaging the adjacent vertebralbodies. While not so limited, a preferred embodiment of the presentinvention is adapted to receive a cancellous lag screw, having a headportion incapable of passing out of the implant so as to be containedtherein. It is further preferred that the implant of the presentinvention are adapted to receive a lock for locking the oppositevertebrae engaging bone screws to the implants. Still further preferredis that the implants of the present invention are adapted to receive thelag screws and screw locks so as to allow the locks to function toprevent the backing out of the screws while, nevertheless allowing forsome angular motion of the screws relative to the implants.

FIG. 9 shows a front view of an embodiment of the present invention withimplants 100, 200 properly implanted across the disc space betweenadjacent vertebral bodies V₁ and V₂. Openings 144, 244 of implants 100,200, respectively, allow for vascular access through trailing ends 104,204 of implants 100, 200 and for bone growth therethrough. Trailing ends104, 204 have common openings 126, 226 and situated essentially therein,are threaded openings 148, 248 for receiving an implant driver. Theimplant driver has a distal end for a complimentary fit within commonopenings 126, 226 and therethrough a rotatable threaded member forthreading into openings 148, 248, respectively. Openings 126,226 alsoare adapted to receive a screw device to link the implant to otherimplants, to a staple, or to receive a locking screw to lock boneengaging screws to the implant as disclosed in Michelson U.S. patentapplication Ser. No. 08/926,334 incorporated herein by reference. Commonopening 126, 226 also may have therein opposed and divergently angledopenings 136, 236 and 136′, 236′ adapted to receive opposed vertebralbone engaging screws. Bone screw receiving openings 136, 236 and 136′,236′ preferably may have circumferentially around them retaining seats140, 240 and 140′, 240′ adapted to receive and to block the passage ofthe heads of screws to be inserted therethrough. Retaining seats140,240, and 140′, 240′ may also be flanged.

As shown in the preferred embodiment of the present invention, trailingends 104 and 204 of implants 100 and 200, respectively, preferably arerotationally asymmetrical about the longitudinal axes of the implantssuch that the designated medial side of each of the implants has alength greater than the lateral sides of the same implants. Trailingends 104, 204 preferably are structured to have a lesser length alongtheir lateral sides than through the mid-longitudinal axis and arepreferably contoured so as to sit on the anterior rims of the vertebralbodies without protruding dangerously therefrom as set forth in pendingMichelson application Ser. No. 09/263,266 incorporated herein byreference. In another embodiment of the present invention, the trailingends of the implants can have a maximum length along themid-longitudinal axis greater than the length along either of the medialand lateral sidewalls so that the bone screw receiving holes can beoriented towards the adjacent vertebral bodies in half rotationincrements rather than requiring a full rotation. While for implant 100this would require no other modification than as described for thetrailing end, in regard to implant 200 each of the lateral and medialside walls would have to be relieved, as shown in FIG. 4B for example,to allow for the receipt of the perimeter of implant 100 within themaximum perimeter of implant 200.

FIGS. 10-14 show a series of steps useful for discussing a method of useof the present invention implants. Methods for inserting spinal implantsare discussed in part in issued and pending patent applications toMichelson U.S. Pat. Nos. 5,593,409, 5,741,253, 5,484,437, Ser Nos.08/396,414, and 08/480,904, incorporated by reference herein. The discspace to be used is preferably, but not necessarily, distracted tooptimal height and the vertebral bodies preferably, but not necessarily,properly aligned. A pair of overlapping bores are then formed across thedisc space with a bone removal device such as a drill having a diametergreater than the height of a distracted disc space such that arc-shapedportions of bone are removed from each of the vertebral bodies adjacentthe disc space to be fused. The overlapping bores are generally orientedfrom anterior to posterior and preferably stop short of the spinalcanal.

A bone removal device such as a drill or mill that may be conical can beutilized to complement the tapered configuration of the implant body. Asshown in FIG. 10, however, in a preferred method a generally cylindricaldrill DR or end mill is utilized to create a generally cylindrical borefor receiving the implants. When a pair of generally cylindricaloverlapping bores, preferably but not necessarily, having a diametergenerally corresponding to that of the root diameter of the implantproximate the leading end are formed as per FIG. 3A, the implants willcome to be positioned such that the combined width of the implants attheir leading ends will be less than the combined width of implants attheir trailing end. That is, the implants will be angled in towards eachother from anterior to posterior. This has the further benefit ofswinging the junction of the lateral side walls and trailing endsfurther inward and away from escaping the anterior vertebral cortex,thereby avoiding protrusion of the lateral side wall to trailing endjunctions and allowing for the installation of larger and longerimplants than might otherwise be possible.

As has been taught by Michelson in the above identified applications andpatents incorporated by reference herein, the disc space may bedistracted in any number of ways and held distracted during the boreformation portion of the procedure. Some of the preferred ways includethe use of long distractors, short distractors, and extended outersleeves having distractor members for placement within the disc spaceand between the adjacent vertebral bodies as described by Michelson inthe above described applications and patents incorporated by referenceherein. Other distractors such as those which attach to the vertebralbodies as by pins or screws might also be useful for the presentintended purpose.

While surgery may be performed through a single bore first, in apreferred embodiment both bores are created in overlapping fashion priorto the insertion of the first implant which in this example is implant200. Implant 200 is affixed to an implant driver which driver preferablyengages the implant at trailing wall 204 by interdigitating with implant200 and further binding to implant 200 by a thread such that it ispossible both to rotate implant 200 in either direction and to push orpull simultaneously. While that may be achieved by having a driver whichinterdigitates with any of the openings into or through rear wall 204and having a rotatable portion for threading into threaded opening 248the present invention is not so limited and may include any driveruseful for the intended purpose.

After implant 200 is fully seated with medial side wall 228 orientedimmediately toward the disc space, a complementary implant 100 isinserted by allowing it to rotate within the maximum circumference ofimplant 200. Pre-tapping the bores formed across the disc space prior tothe insertion of the implants does not deviate from the presentteaching. In a preferred embodiment, pre-tapping is not required ascertain preferred embodiments of the present implants are tapered fromtheir trailing to their leading ends and the leading ends haveparticularly significant thread heights making their ability to threadthemselves into the bone particularly effective.

FIGS. 11-14, show openings at the trailing end of the implant forreceiving opposed screws that may be oriented from the implant into eachof the adjacent vertebral bodies. These screws enter the implant throughthe trailing end and the threaded shafts of the screws pass throughopenings in the opposite upper and lower vertebral body engagingsurfaces of the implants. Shown in FIG. 11 is a cut away through implant100 of FIG. 9. This is a cross section through the mid-longitudinal axisof implant 100 and the adjacent vertebral bodies. FIG. 12 shows a screwdriver 450 driving a bone screw 500 through common opening 126, bonescrew receiving hole 136′ and out opening 124′ through lower vertebraeengaging surface 134′ into adjacent vertebral body V₂. The presentinvention includes the use of any bone screws for this describedpurpose. In preferred embodiments, structure is provided to block thebone screws from disengaging from the implant or backing out. The screwsmay be rigidly locked to the implant or may be prevented from backingout in a manner that still allows for some relative motion between thescrews and the implant. The latter may be beneficial for anticipatingand allowing for some settling of the vertebral bodies towards the discspace.

In use, as shown in FIG. 12, the driver 450 is assembled to the screw500 thereby compressing the head portion of the screw. The screw is thenintroduced through the trailing end of the implant and directed into thebody of one of the adjacent vertebrae passing out of an opening adaptedfor that purpose in one of the opposite vertebrae engaging surfaces ofthe implant. The head of the screw 500 is too large to pass through theopening in the implant, and yet is free to spin against the implantitself making it possible to lag the screw, or that is to draw the bodyof the vertebra to the implant and to generate compressive load betweenthe implant and the vertebral body. As shown in FIG. 13, when the screwhas been fully seated and the driver removed, the head of the screw isfree to reexpand, thereby locking it to the implant. As mentioned, thepresent invention includes the use of any opposed vertebrae engagingbone screws, such that at least one each of said screws binds theimplant to each of the adjacent vertebral bodies. In a preferredembodiment, the screws are prevented from backing out of the implant.Screws may be locked directly to the implant such that they are rigidlyattached thereto, or may be capable of some movement relative to theimplant so as to allow for variation in screw positioning and/orsettling of the vertebrae, and yet be prevented from backing out.

While the present invention is shown in a preferred embodiment as bothhighly perforate and substantially hollow, the implant could comprise agenerally porous or cancellous material allowing for the growth of bonein continuity from vertebrae to vertebrae through the implant. Thepresent invention implants can comprise of any material that isbiocompatible and structurally suitable for construction of theseinterbody spinal fusion implants and consistent with the growth of bonefrom vertebral body to vertebral body through the implants. To that end,materials which would be satisfactory might include implant qualitymetals such as surgical quality titanium and its alloys, cobalt chromeor other metals useful for this purpose, cortical bone and in particularhuman cortical bone such as that which might be obtained from one of thetubular long bones of a human body, ceramics, plastics and compositematerials including those incorporating carbon fibre; and such implantsmay further comprise, contain, be treated with or coated with osteogenicmaterials other than bone for the purpose of achieving spinal fusion.Such materials would include but not be limited to bone morphogeneticprotein, ossification inducing proteins, and genes coding for theproduction of bone. Further, the implants may include at least in partor wholly materials bioabsorbable by the human body, which by way ofexample only may include plastics selected from the lactide, lactonefamily, polylactide, polylactone family, glycolic acid derivatives andso forth.

The implants of the present invention may be coated with, comprised of,be used in combination with, or have a hollow for containing bone growthpromoting materials, including but not limited to, bone morphogeneticproteins, hydroxyapatite, and genes coding for the production of bone.The implants of the present invention can be formed of a material thatintrinsically participates in the growth of bone from one of adjacentvertebral bodies V to the other of adjacent vertebral bodies V.

While the specific preferred embodiments the implants of the presentinvention have been described, again the present invention is not solimited. The present invention includes any interbody spinal fusionimplants embodying the present teachings including implants havingopposite surfaces with a thread, portion of a thread, or generallyconcentric fins used to penetrably engage the substance of the vertebralbodies when the implants are rotated about their longitudinal axesapproximately 90 degrees or more.

The implants of the present invention are preferably hollow with aplurality of openings through the various surfaces of the implant incommunication with the implant hollow. The present invention includeseither fewer, or more openings so long as each of the opposite vertebraeengaging surfaces of the implant have at least one opening incommunication with the other so as to allow for the growth of bone incontinuity from vertebral body to adjacent vertebral body through theimplant.

There is disclosed in the above description and the drawings implants,which fully and effectively accomplish the objectives of this invention.However, it will be apparent that variations and modifications of thedisclosed embodiments may be made without departing from the principlesof the invention or the scope of the appended claims.

1. An interbody spinal fusion implant for insertion across a disc spacebetween two adjacent vertebral bodies of a human spine, said implantcomprising: a leading end, a trailing end opposite said leading end anda length therebetween; a hollow interior; opposite arcuate portionsadapted for placement toward and at least in part within the adjacentvertebral bodies and having a distance therebetween defining an implantheight greater than the normal height of the disc space, each of saidopposite arcuate portions having at least one opening communicating withone another and said hollow interior; at least a portion of a threadformed on the exterior of each of said opposite arcuate portions forpenetrably engaging the adjacent vertebral bodies; a leading supportwall connecting said opposite arcuate portions at said leading end; atrailing support wall connecting said opposite arcuate portions at saidtrailing end; and a lateral side and a medial side opposite said lateralside, said lateral side including a side support wall connecting saidopposite arcuate portions, said medial side including an openingextending from one of said opposite arcuate portions to the other ofsaid opposite arcuate portions and extending from said leading supportwall to said trailing support wall along a majority of the length ofsaid implant, each of said arcuate portions terminating at an edge atsaid medial side, said opening in said medial side having a height in aplane perpendicular to the length of said implant, said edges beingspaced apart a distance equal to the height of said opening in saidmedial side, said arcuate portions each having a surface from said edgeto said hollow interior at said opening on said medial side, each ofsaid surfaces being curved from said edge to said hollow interior in theplane perpendicular to the length of said implant.
 2. The implant ofclaim 1, wherein said opposite arcuate portions form at least a portionof a cylinder along the length of said implant.
 3. The implant of claim1, wherein said opposite arcuate portions form a generallyfrusto-conical shape.
 4. The implant of claim 1, wherein said trailingend of said implant has a perimeter, said perimeter of said trailing endproximate said medial side being configured to receive at least aportion of a perimeter of a trailing end of a second implant when thesecond implant is placed in close proximity to said implant.
 5. Theimplant of claim 1, wherein said implant has an outer diameter having acircumference, said medial side adapted to receive at least a portion ofa circumference of an outer diameter of a second implant.
 6. The implantof claim 1, wherein the opening between said leading and trailing endshas a maximum dimension in a direction along the mid-longitudinal axisof said implant.
 7. The implant of claim 1, wherein said side supportwall and said opposite arcuate portions form parts of the same circle.8. The implant of claim 1, wherein said leading and trailing supportwalls each include a concave portion at said medial side and a convexportion at said lateral side.
 9. The implant of claim 1, wherein saidimplant is at least in part bioabsorbable.
 10. The implant of claim 1,wherein said implant is in combination with a bone growth promotingmaterial.
 11. The implant of claim 1, wherein said implant has a centrallength from said leading end to said trailing end, said hollow interiorbeing unobstructed along the entire central length of said implant. 12.An interbody spinal fusion implant for insertion across a disc spacebetween two adjacent vertebral bodies of a human spine, said implantcomprising: a leading end, a trailing end opposite said leading end anda length therebetween; a hollow interior; opposite arcuate portionsadapted for placement toward and at least in part within the adjacentvertebral bodies and having a distance therebetween defining an implantheight greater than the normal height of the disc space, each of saidopposite arcuate portions having at least one opening communicating withone another and said hollow interior; at least a portion of a threadformed on the exterior of each of said opposite arcuate portions forpenetrably engaging the adjacent vertebral bodies; a lateral side and amedial side opposite said lateral side, said lateral side including aside support wall connecting said opposite arcuate portions, said medialside of said implant including an opening extending from one of saidopposite arcuate portions to the other of said opposite arcuate portionsand extending from said leading end to said trailing end of said implantalong a majority of the length of said implant, each of said arcuateportions terminating at an edge at said medial side, said opening insaid medial side having a height in a plane perpendicular to the lengthof said implant, said edges being spaced apart a distance equal to theheight of said opening in said medial side; a leading support wallconnecting said opposite arcuate portions and said side support wall atsaid leading end of said implant, said leading support wall having aconcave portion at said medial side of said implant adjacent saidopening, said concave portion of said leading support wall and saidopposite arcuate portions having substantially the same radius ofcurvature; and a trailing support wall connecting said opposite arcuateportions and said side support wall at said trailing end of saidimplant, said trailing support wall having a concave portion at saidmedial side of said implant adjacent said opening, said concave portionof said trailing support wall and said opposite arcuate portions havingsubstantially the same radius of curvature.
 13. The implant of claim 12,wherein said opposite arcuate portions form at least a portion of acylinder along the length of said implant.
 14. The implant of claim 12,wherein said opposite arcuate portions form a generally frusto-conicalshape.
 15. The implant of claim 12, wherein said trailing end of saidimplant has a perimeter, said perimeter of said trailing end proximatesaid medial side being configured to receive at least a portion of aperimeter of a trailing end of a second implant when the second implantis placed in close proximity to said implant.
 16. The implant of claim12, wherein said implant has an outer diameter having a circumference,said medial side adapted to receive at least a portion of acircumference of an outer diameter of a second implant.
 17. The implantof claim 12, wherein the opening between said leading and trailing endshas a maximum dimension in a direction along the mid-longitudinal axisof said implant.
 18. The implant of claim 12, wherein said side supportwall and said opposite arcuate portions form parts of the same circle.19. The implant of claim 12, wherein said implant is at least in partbioabsorbable.
 20. The implant of claim 12, wherein said implant is incombination with a bone growth promoting material.
 21. The implant ofclaim 12, wherein said implant has a central length from said leadingend to said trailing end, said hollow interior being unobstructed alongthe entire central length of said implant.
 22. A pair of non-identical,complementary, interbody spinal fusion implants for insertion across adisc space between two adjacent vertebral bodies of a spine, said pairof implants comprising: a first spinal fusion implant having a leadingend, a trailing end opposite said leading end, a length, and a hollowinterior therebetween, said first implant having opposite arcuateportions adapted for placement toward and at least in part within theadjacent vertebral bodies and having a distance therebetween defining afirst implant height greater than the normal height of the disc space,each of said opposite arcuate portions having at least one openingcommunicating with one another and said hollow interior, at least aportion of a thread formed on the exterior of each of said oppositearcuate portions for penetrably engaging the adjacent vertebral bodies,a leading support wall connecting said opposite arcuate portions at saidleading end, a trailing support wall connecting said opposite arcuateportions at said trailing end, said first implant having a lateral sideand a medial side and having a distance therebetween defining a firstimplant width transverse to said first implant height, said firstimplant width being less than said first implant height along at least aportion of the length of said first implant, said lateral side of saidfirst implant including a side support wall connecting said oppositearcuate portions, said medial side of said first implant including anopening extending from one of said opposite arcuate portions to theother of said opposite arcuate portions and extending from said leadingsupport wall to said trailing support wall along a majority of thelength of said first implant, each of said arcuate portions terminatingat an edge at said medial side, said opening in said medial side havinga height in a plane perpendicular to the length of said first implant,said edges being spaced apart a distance equal to the height of saidopening in said medial side, said leading support wall having a concaveportion at said medial side of said first implant adjacent said opening,said concave portion of said leading support wall and said oppositearcuate portions having substantially the same radius of curvature; asecond spinal fusion implant having a leading end, a trailing endopposite said leading end, a length, and a hollow interior therebetween,said second implant having opposite arcuate portions adapted forplacement toward and at least in part within the adjacent vertebralbodies and having a distance therebetween defining a second implantheight greater than the normal height of the disc space, each of saidopposite arcuate portions having at least one opening communicating withone another and said hollow interior, at least a portion of a threadformed on the exterior of each of said opposite arcuate portions forpenetrably engaging the adjacent vertebral bodies, a leading supportwall connecting said opposite arcuate portions at said leading end, atrailing support wall connecting said opposite arcuate portions at saidtrailing end, said second implant having a lateral side and a medialside and having a distance therebetween defining a second implant widthtransverse to said second implant height, said medial side and saidlateral side of said second implant each including a side support wallconnecting said opposite arcuate portions, said first implant beingconfigured to receive at least a portion of said medial side of saidsecond implant in said opening of said medial side of said firstimplant; and said first and second implants having a combined width whensaid medial side of said second implant is received by said medial sideof said first implant being less than a sum of said first implant widthand said second implant width.
 23. The implants of claim 22, whereinsaid opposite arcuate portions of said first and second implants form atleast a portion of a cylinder along the length of said first and secondimplants.
 24. The implants of claim 22, wherein said opposite arcuateportions of said first and second implants form a generallyfrusto-conical shape.
 25. The implants of claim 22, wherein saidtrailing end of said first implant has a perimeter, said perimeter ofsaid trailing end of said first implant proximate said medial side ofsaid first implant being configured to receive at least a portion of aperimeter of a trailing end of said second implant when said secondimplant is placed in close proximity to said first implant.
 26. Theimplants of claim 22, wherein said first implant has an outer diameterhaving a circumference, said medial side of said first implant adaptedto receive at least a portion of a circumference of an outer diameter ofsaid second implant.
 27. The implants of claim 22, wherein the openingbetween said leading and trailing ends of said first implant has amaximum dimension in a direction along the mid-longitudinal axis of saidfirst implant.
 28. The implants of claim 22, wherein said side supportwall of said first implant and said opposite arcuate portions of saidfirst implant form parts of the same circle.
 29. The implants of claim22, wherein said first and second implants are at least in partbioabsorbable.
 30. The implants of claim 22, wherein said implants arein combination with a bone growth promoting material.
 31. The implantsof claim 22, wherein said arcuate portions of said first implant eachhave a surface from said edge to said hollow interior at said opening onsaid medial side, each of said surfaces being curved from said edge tosaid hollow interior in the plane perpendicular to the length of saidfirst implant.
 32. The implants of claim 22, wherein said first implanthas a central length from said leading end to said trailing end, saidhollow interior being unobstructed along the entire central length ofsaid implant.